Co-Action of Ionic Liquids with Alternative Sorbents for Removal of Reactive Azo Dyes from Polluted Wastewater Streams

Round 1

Reviewer 1 Report

Co-Action of Ionic Liquids with Alternative Sorbents for Removal of Reactive Azo Dyes from Polluted Wastewater Streams

In this work, authors perform the removal of a reactive dye known as Reactive Black 8 (RB8) from wastewater using biochar and bentonite with ionic liquids. The work is a great example of the utilization of carbon materials and furthermore, waste, in high value-added applications for a sustainable future. However, I do believe that the text needs to be improved to give the manuscript the attention it needs, and to highlight its importance in the field. Below there is some suggestions of schemes that could be introduced to clarify some important aspects of the work, and summary some of the conclusions obtained. Therefore, I support the publication of this manuscript after moderate editing and after the revisions below:

•     Page 2, line 54: Authors mention biological methods for treating dyes-loaded wastewater but no examples are given.

•     Equation 1 should be moved to Supplementary Information, while the structure of the ionic liquids should be moved to the main text. Also, structure of biochar or BT alone should be included so readers can have an idea of the materials being used in this work.

•     Do the authors believe that any chemical reaction is taking place between the IL and biochar? If not, how do the authors can prove that?

•     Page 5, line 182: “…only BC doses higher than 40 g/L provided decolorization efficiency.” Does this mean that the biochar completely soluble in the aqueous environment provided?

•     In the introduction, authors should include a scheme to summarize the research being performed in this article.

•     In page 10, the authors list a number of mechanisms that can be used to explain the relationship between the removal of contaminants and the sorbents. However, they should also add schemes and figures to explain and differentiate these interactions - are all of those happening at the same time? Is the removal of contaminants directed mainly by one of those mechanisms?

Authors should go through the text for checking typos and language mistakes.

Author Response

Dear Reviewer,

we thank you for such an opinion and for valuable comments. We have done our best to reflect all your queries and recommendations, incorporating your comments in the new version of article. The introduction and results and discussions parts were improved, please see chapters 1 and 3. All these changes/improvements in new version of article are marked yellow. We also edited the typos and language mistakes.

• Thank you for this suggestion. We revised in an article and described mentioned biological treatment and provided some examples, please, see Introduction part.
• For better clarity of article, we moved the structures of ILs to the main text, see Introduction part. We also included basic description of chemical a physical characteristic of BC as well as BT with references on the structures of mentioned sorbents, see also Introduction part.
• Thank you for interesting comment. No, we do not believe that the biochar does not react with ILs (BAC). As we observed in our previous study [32] and even observed in this study (see section 3.3 – the explanation of adsorption mechanisms – point c), the ILs such as BAC can be well adsorbed on the surface of the BC. It also corresponds with other articles in which researchers described good affinity of quaternary ammonium compounds to the surface of rather non-polar carbonaceous sorbents [38,59]. On the other hand, it is evident that the ion-pair formation reaction is more rapid compared with sorption of BAC on surface of biochar and that the formed ion-pairs (which are less soluble in water, see log Pow results in Fig. S6 in Supplementary Materials) proved better affinity to the low polar surface of BC than BAC alone. This proposition was described and proved in our earlier studies, you can see refs. https://doi.org/10.3390/w15183178, https://doi.org/10.1016/j.talanta.2023.125073, https://doi.org/10.5772/intechopen.92760 or https://doi.org/10.1515/chempap-2015-0225.
• No, the fact that only BC doses higher than 40 g/L provided satisfactory decolorization efficiency (>70%) means that lower dosages than 40 g/L are not able to bind sufficient quantity of tested dye RB8, please see Fig. 3. For example, the dosages of BC 30 g/L provided removal efficiency of RB8 ca. 10% lower than discussed 40 g/L of BC from tested aq. solutions of RB8. However, we are aware that biochar potentially could contains water-soluble impurities, see, e.g., following article https://doi.org/10.5539/jas.v13n1p1. However, this is a negligible fact that has little or no effect on the resulting dye sorption. Despite, we better explained this obviously misunderstanding fact in revised article, see section 3.1. So, we thank you for pointing this out.
• We include a process cycle layout to clarify the whole removal process. The experimental scope of RB8 separation cycle layout using alternative sorbents in the presence/absence of benzalkonium chloride is illustrated and presented in new Scheme S1 and Figure S3 in the Supplementary Materials. However, based on suggestions of other reviewers, we rather included mentioned scheme and Figure in section 2.3 than in the Introduction section. Moreover, we also include a scheme with explanation of adsorption mechanisms, see Scheme S2 in the Supplementary Materials
• Thank you for pointing out the lack and insufficiently explained adsorption mechanisms. In the revised article, we clarified the process of separation mechanisms, and we also included the scheme that depicts mentioned mechanisms, please, see section 3.3. and Scheme S2 in Supplementary Materials. (Based on observed rapid ion exchange (see Fig. 5 above and also refs. [32,44,45]) and non-polar character of formed ion pairs (see Fig. S6 in Supplementary Materials and also refs. [32,44,45]), we suppose that the main separation mechanism is the formation of ion-pairs and sorption/entrapment of these ion-pairs using alternative sorbents.)

Reviewer 2 Report

1. In Section 1. It is advisable to include the recent papers on the basic characteristics of BC and BT such as the physical and chemical characteristics, and applications.

2. It is recommended that the specifications of adsorbents in Table 1 should be tested by oneself rather than citing literature.

3. Please confirm again that the results of Eq. 2 in Section 2.4 are not discussed in the Results (Section 3).

4. The authors please confirm again that the title of Section 3 should be “Results and discussion”. And Section 5 should be replaced by Section 4.

5. In Section 3. It is not found for Figure 8 in this article. Moreover, Figure 8 are also not illustrated in the text of this article.

6. In the line285 - 289 of Section 3.3. The authors should clearly illustrate the reasons for removal efficiency: BT < BC < MTM < PAC < BT+BAC < BC+BAC.

7. In the line 322 - 349 of Section 3.3. The adsorption mechanism is not clear. The author should use instrumental test results as evidence to explain the adsorption mechanism

8. It requires some more critical data including XRD, SEM, FTIR, as well as surface area, pore volume and pore size analysis, due to data is not enough.

Author Response

Dear Reviewer,

we thank you for such an opinion and for valuable comments. We have done our best to reflect all your queries and recommendations, incorporating your comments in the new version of article. The results and discussions part were improved, please see chapter 3. All changes in new version of article are marked yellow.

1. Thank you, based on your great suggestion, we included basic description of chemical and physical characteristic of BC as well as BT with references on the structures of mentioned sorbents and the possibly applications of these sorbents, see Introduction part.

2. Yes, of course, you are right, it is usual to provided tested specifications of adsorbents rather than citing literature. However, in our previous work [32] we briefly described the production and tested specifications of alternative sorbent biochar. Moreover, tested sample of biochar (supplied from the research group from Institute of Chemical Process Fundamentals of the Czech Academy of Sciences) was detailed described and characterised in several research articles [32,23,24]. We also included these references in revised form of article, see section 2.2. For more specifications and analysis, please see mentioned references. The other used adsorbents were specified well by the suppliers or other authors [47-50]. Thus, we allowed to refer to these previous references for the sake of simplifying the current article and we cite the literature rather than provided testified specifications.

3. The results of Eq. 2 in Section 2.4 are discussed in the Results and discussions (Section 3.2) and the results of P_ow are provided in the Supplementary materials – Fig. S6.

4. Thank you for pointing out our mistakes, you are right. It is revised in the actual version of our manuscript.

5. Thank you for pointing out our mistake, you are right. It was revised in article.
6. Yes, you are right, the description of the reasons of reached removal efficiency order (BT < BC < MTM < PAC < BT+BAC < BC+BAC) more clarified the provided results. We included this description, see section 3.3.

7.+8. Thank you for valuable comments, you are right. As we mentioned in our previous article [44], adsorption mechanisms can be potentially established by several analytical methods, i.e. FTIR or other analytical methods. Therefore, BC samples before and after adsorption of RB8 were characterized using XRF and FTIR. XRF measurements approved significantly higher content of chromium in BC samples saturated with RB8 dye (please, see Figures S5 and S9 in Supplementary Materials). Unfortunately, FTIR spectra measured by the DRIFT technique provided no detectable signals of adsorbed dye RB8. Another analysis such as SEM, pore volume, etc. of tested sample of biochar were provided in several articles [23,24] of research group from the Institute of Chemical Process Fundamentals of the Czech Academy of Sciences (the suppliers of the sample of BC), see section 2.2. The other suggested analysis such as surface area or pore volume are available in Table 1 or in our previous work [32]. However, more extensive study focused on this topic could be the subject of our future work.

Reviewer 3 Report

The authors talk about Co-Action of Ionic Liquids with Alternative Sorbents for Removal of Reactive Azo Dyes from Polluted Wastewater Streams and is a good research wherein a process development is discussed for azo dye separation from water. 

One suggestion is to include a process cycle layout which would give the readers more clarity on the entire process. 

Another suggestion is to include and cite articles that discuss a) other research on similar field https://doi.org/10.1016/j.ecoenv.2021.113160 and b) environment impact of industrial waste streams https://doi.org/10.3390/su152215758

Author Response

Dear Reviewer,

we thank you for such an opinion and for valuable comments. We have done our best to reflect all your queries and recommendations, incorporating your comments in the new version of article. We include a process cycle layout to clarity on the entire process. The experimental scope of RB8 separation cycle layout using alternative sorbents with presence/absence of benzalkonium chloride is illustrated and presented in Scheme S1 and Figure S3 in the Supplementary Materials, see also section 2.3. Moreover, we also include a scheme with explanation of adsorption mechanisms, see Scheme S2 in the Supplementary Materials.

We also thank you very much for the recommendation of references, ref. a) https://doi.org/10.1016/j.ecoenv.2021.113160 was very valuable for our current article, and we believe that the including of this reference improved the quality of our article. All changes in new version of article are marked yellow.

Best regards,

the Authors

Reviewer 4 Report

The manuscript titled “Co-Action of Ionic Liquids with Alternative Sorbents for Removal of Reactive Azo Dyes from Polluted Wastewater Streams” provides a valuable study of the removal of azo dye RB8 from wastewater using biochar or bentonite in combination with ionic liquids, specifically benzalkonium chloride or Aliquat 336. I recommend its publication in Applied Sciences after addressing the following comments:

1. Line 84: Please provide a brief definition of ionic liquids. Additionally, as ILs have been extensively studied for decolorization of dye effluents, especially with aqueous two-phase systems, please include a concise overview of these findings.

2. Line 260: Please provide a brief description of the impregnation methods and the co-action of sorbents with ILs.

3. Please discuss the following in the text:

-Is there a potential for regenerating the used adsorbents, and is this process challenging, particularly considering that it involves a combination of two materials, IL+BC or IL+BT?

- Considering that BAC is soluble in water, does it persist partially in the water after treatment? How harmful is it to the environment?

Author Response

Dear Reviewer,

we thank you for such an opinion and for valuable comments. We have done our best to reflect all your queries and recommendations, incorporating your comments in the new version of article. All changes in new version of article are marked yellow.

1. Thank you for valuable point. We provided a brief description of ionic liquids and their application in separation of dye effluents, see Introduction part. However, for further details, please see our earlier study focused on application of ILs for separation of acid azo dyes [45]. We also refer this citation [45] in current article.

2. We also improved the description of impregnation methods, please, see section 3.3. Moreover, we provided the scheme of RB8 separation cycle layout using alternative sorbents with presence/absence of benzalkonium chloride, see Scheme S1 and Figure S3 in the Supplementary Materials. We also include a scheme with explanation of adsorption mechanisms based on in-situ impregnation methods of sorbents using BAC, see Scheme S2 in the Supplementary Materials.

3. Thank you for interesting comments, both of these suggestions with respective references were discussed and incorporated in a new version of article. Please, see revised article – sections 3.2 and 3.4.

Best regards,

the Authors

Round 2

Reviewer 2 Report

The authors have addressed all the concerns from the reviewer. This manuscript is full enough to publish in Applied Sciences.